Trimethylation enhancement using diazomethane (TrEnDi) is a derivatization technique that significantly enhances the signal intensity of glycerophospholipid species in mass spectrometry (MS) and tandem mass spectrometry (MS/MS) analyses. Here, we describe a novel apparatus that is able to conduct in situ TrEnDi (iTrEnDi) by generating and immediately reacting small amounts of gaseous diazoalkane with analyte molecules. iTrEnDi allows complete and rapid methylation of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidic acid (PA), and sphingomyelin (SM) in a safe manner by removing any need for direct handling of dangerous diazoalkane solutions. iTrEnDi-modified PC ([PCTr]+) and PE ([PETr]+) showed similar sensitivity enhancements and fragmentation patterns compared to our previously reported methodology. iTrEnDi yielded dimethylated PA ([PATr]), which exhibited dramatically improved chromatographic behavior and a 14-fold increase in liquid chromatography MS (LCMS) sensitivity compared to unmodified PA. In comparison to in-solution-based TrEnDi, iTrEnDi demonstrated a modest decrease in sensitivity, likely due to analyte losses during handling. However, the enhanced safety benefits of iTrEnDi coupled with its ease of use and capacity for automation, as well as its accommodation of more-reactive diazoalkane species, vastly improve the accessibility and utility of this derivatization technique. Finally, as a proof of concept, iTrEnDi was used to produce diazoethane (DZE), a more-reactive diazoalkane than diazomethane. Reaction between DZE and PC yielded ethylated [PCTr]+, which fragmented via MS/MS to produce a high-intensity characteristic fragment ion, enabling a novel and highly sensitive precursor ion scan.